MC68HC05L28 Datasheet, PDF (24/156 Pages) Motorola, Inc – Flexible general-purpose microcomputer

2.1.1 MC68HC05L28 modes of operation
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2.1.1.1 Single chip mode
This is the normal operating mode of the MC68HC05L28 and the MC68HC705L28. In this mode
the device functions as a self-contained microcomputer (MCU) with all on-board peripherals,
including two 8-bit I/O ports and one 6-bit I/O port, available to the user. All address and data
activity occurs within the MCU.
Single chip mode is entered on the rising edge of RESET if the voltage level on the IRQ0 pin is
within the normal operating range.
Warning: In the MC68HC705L28, all vectors are fetched from EPROM in single chip mode;
therefore, the EPROM must be programmed (via the bootloader mode) before the
device is powered up in single chip mode.
2.1.1.2 RAM bootloader mode
The RAM bootloader mode for the MC68HC05L28 and MC68HC705L28 allows the user to
perform simple load and execute instructions in RAM. To make use of this feature a circuit board
should be constructed as shown in Figure 2-1. Correct configuration of port pins PB2 and PB3
enables loading of a user program into RAM for execution.
The RAM bootloader is selected when the device is put into bootloader mode with PB2 held high.
If PB3 is low, the program counter is set to $0081 and a previously loaded RAM program can be
executed. If PB3 is high at reset a program is serially loaded from PA0 into the RAM and executed
from $0081.
The first byte to be loaded is the count byte which should hold the program length plus the count
byte. Therefore, for a program length of $30, the count should equal $31. The maximum program
size, including the count byte is 254 bytes ($FE), as two bytes must be left for the stack during
download. The format of data for the RAM bootloader mode is 9600 baud, 8-bit, no parity, 1 start,
1stop (for 2 MHz bus speed).
In the RAM bootloader mode interrupt vectors are mapped to pseudo-vectors in RAM (for
MC68HC05L28 vectors see Table 2-4 and for MC68HC705L28 vectors seeTable 2-4). This allows
programmers to use their own service-routine addresses. Each pseudo-vector is allowed three
bytes of space, rather than the two bytes for normal vectors, because an explicit jump (JMP)
opcode is needed to cause the desired jump to the user’s service-routine address.
MOTOROLA
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MODES OF OPERATION AND PIN DESCRIPTIONS
TPG
MC68HC05L28